My invention relates to a video playback system which in practice may take the form of what is known as a video tape recorder, by which I mean the device having provisions for both recording video signals on, and playing them back from, magnetic tape packaged in cassette or cartridge form. More particularly, my invention pertains to a system in a video tape recorder or the like for the reduction of noise from frequency modulated picture signals being reproduced from the record medium.
The quality of picture information reproduced by the video tape recorder depends largely upon its signal to noise ratio (SNR) and frequency characteristic. The SNR is a measure of the freedom of the image from noise whereas the frequency characteristic is a measure of image resolution. Video signals might be faithfully reproduced to a high frequency range through improvement of the frequency characteristic. But then the absolute amount of high frequency noise contained in the video signals would also increase. Therefore, for picture reproduction from the same tape, the improvement of both SNR and frequency characteristic is self contradictory.
Noise is produced in video tape recorders by the magnetic tape traveling around the rotary head assembly as well as by their circuit means. I know two conventional approaches to the problem of how to reduce such noise signals.
One such conventional approach is the preemphasis/deemphasis technique. The magnitude of a higher frequency range of a picture signal is increased (preemphasized) at the time of recording and reduced (deemphasized) at the time of reproduction, so that the picture signal is reproduced in its original form. There are, however, many noise components in video playback systems that cannot be eliminated by this known technique. A nonlinear emphasis method has therefore been developed in which the desired frequency range is preemphasized to variable degrees depending upon the varying strengths of the picture signal. This known method is objectionable because of the complex and expensive circuitry required.
The other conventional approach is known as a signal delay noise reduction system. This system employs a field memory or a delay line for determining the similarity or dissimilarity of the successive fractions, each corresponding to one horizontal scanning period, of the picture signal being reproduced. Signal components not closely related with others are removed.
Admittedly, the signal delay noise reduction system is very effective in the case where the reproduced image involves little motion. The trouble is that the similarity of the successive picture signal fractions decreases in inverse proportion to the rapidity of motion in the image represented by that picture signal. This known system hardly works with the picture signal representing rapid motion. I am aware that attempts have been made to defeat this weakness by detecting the varying degrees of motion in the picture signal and adjusting the noise reduction level to the detected degrees. Here again, however, the resulting circuitry is very complex and expensive.